Method for electrostatic painting



June 1, 1965 w. s. PETTIGREW ETAL 3,186,864

METHOD FOR ELETROSTAT IC PAINTING Filed Feb. 19, 1962 VIBRATOR POWER 8 UPPLY VIBRATOR Q) (7 1kg ATTORNEY United States Patent 3,186,864 METHODFGR ELEQTRGSTATIE PAH' ITING William S. Pettigrew, Fleasant Ridge, andEugene W.

Christen, Birmingham, Mich, assignors to General Meters Corporation,Detroit, Mich, a corporation of Delaware Filed Feb. 19, 1962, Ser. No.174,072 4 Claims. (Cl. 117-9342) This invention relates to electrostaticcoating and particularly to a method and apparatus utilizing an endlessbelt as an atomizing means.

It has been previously known in the art of electrostatic coating toatomize a liquid material and to subject the atomized material to anelectrical field which terminates on a workpiece for charging thematerial and de positing it on the workpiece. Various methods have beendevised to atomize the material such as conventional air spray guns andrapidly spinning discs. The air spray guns have the advantage ofthrowing the atomized liquid in a desired direction, but inherentlyreduce the effectiveness of the electrostatic field by introducingstrong air currents which tend to blow a large portion of the atomizedmaterial past the workpiece thereby wasting the paint. The spinningdiscs, on the other hand, atomize the coating material by centrifugalforce and cause relatively little wind. An important disadvantage of thediscs, however, is that the coating material comes off in all directionswith respect to the axis of the disc or, in other words, the spray isnot directional. This type of atomizer is not practical for use in themost common type of painting application; that is, where the material tobe painted moves on a straight line conveyor past the paintingequipment.

The aim of the present invention is to provide a centrifugal atomizerwhich forms a spray pattern which is directional in nature. Theinvention is carried out by providing an endless belt passing at highspeed around a roller and means for applying coating material to thebelt whereby the material will be atomized by centrifugal force as thebelt passes around the roller. The atomized material will be thrown intoan electrostatic field which assists in carrying the atomized materialto the workpiece.

The above and other advantages of the invention will become moreapparent from the following specification taken in conjunction with theaccompanying drawings in which like numerals refer to like parts and inwhich:

FIGURE 1 is an elevational view of an electrostatic coating apparatusaccording to the invention;

FIGURE 2 is a plan view of the apparatus of FIG- URE 1;

FIGURE 3 is a schematic illustration of a coating apparatus according toanother embodiment of the invention; and

FIGURE 4 is a schematic illustration of a coating apparatus according tostill another embodiment of the invention.

FIGURES 1 and 2 show painting apparatus comprising an endlesselectrically conductive belt passing around a pair of horizontallyspaced rollers 12 and 14. The rollers are supported by a frame 16 havinga pedestal portion 18 connected to a hydraulic reciprocating mechanism19 on the floor. The roller 12 is driven by a motor 20 while the roller14 is an idler. A paint supply means 22 is mounted on the frame 16 abovethe belt 10 and has a plurality of feed orifices 24 arranged to applypaint to the upper surface of the belt 10. A vibrator 25 mounted on theframe 16 and connected to an air supply by a hose 26 agitates theapparatus so as to aid in attaining a uniform distribution of paint overthe belt 10.

A charging ring 26 is mounted on an outwardly extending portion 28 ofthe frame 16 and defines a loop in front 3,185,864 Patented June 1, 1965"ice of the roller 14. The portion 28 is electrically insulated from theframe 16 by an insulator 30 of any suitable type. A power supply of,say, 100,000 volts has one terminal connected to ground and the otherterminal connected via conductor 32 to the charging ring 26 and alsoconnected through a resistor 34 to the frame 16. Hence, the chargingring will have a potential of about kilovolts and the remainder of theatomizing apparatus will have a lower potential depending upon the sizeof the resistor 34. Preferably the resistor should be selected tomaintain the potential on the belt 10 at about 80,000 volts. However,the device will be operative even if the resistoris eliminated.

A straight line conveyor 40 of conventional structure is arranged tocarry workpieces to be painted past the painting apparatus at a distanceof about 20 inches from the roller 14. The conveyor, and hence, theworkpiece is grounded thereby establishing an electrostatic fieldbetween the workpiece and the charging ring 26.

In operation, the belt motor 20 will be energized to cause the belt totravel at a high speed around the idler roller 14. The paint is appliedto the upper surface of the belt by the paint supply device 22 at a ratesufiicient to form a thin film on the belt. When the paint film reachesthe idler roller 14, it will be thrown off the belt by centrifugal forceand will be projected through the charging ring 26. While the paint ison the charged belt 10, and further as it passes through the chargingring 26, it will become electrically charged so that the electrostaticfield between the charging ring 26 and the workpiece 42 will assist intransporting the paint particles to the workpiece for depositionthereon. The belt 10 should travel at a speed sufficient to atomize thepaint into fine particles. However, this speed will depend upon theviscosity of the paint used as well as the radius of the idler roller14.

Another embodiment of the invention, as schematically depicted in FIGURE3, is the same as that of FIGURES 1 and 2 except that there is no highvoltage connection to the belt 10 and the belt need not be electricallyconductive. In this device the paint is atomized and projected throughthe charging ring 26 as before, but will be charged solely by the effectof the charging ring, and the atomized paint will then be deposited onthe workpiece 42 by an electrostatic field extending between thecharging ring and the workpiece.

The embodiment of the invention shown in FIGURE 4 is likewise similar tothat of FIGURES l and 2 except that the charging ring is omitted and thefull 100,000 volts of the power supply are connected to the conductivebelt 10. Then the electrostatic field will extend from the belt 10 tothe workpiece 42 and the atomization will take place in the sameelectrostatic field which deposits the atomized material on theworkpiece.

In each of the embodiments of the invention described, the atomizingapparatus is mounted on a linear actuator so that the entire apparatuscan be reciprocated vertically and the paint spray can be caused tovertically scan the area through which the workpieces pass.Alternatively, the atomizing apparatus can be made to oscillate about ahorizontal axis to produce the same vertical scanning effect. Inaddition, support member 18 may be made to be rotatable by any suitablemeans, not shown, so that the atomizer can be aimed toward theworkpieces at various' angles or can be made to continuously sweep thework area, if desired.

It will thus be seen that this invention provides a centrifugal atomizerwhich produces a directional spray of coating material without thedeleterious effect of the air stream which accompanies a conventionalair atomizer.

It will be apprecaited that considerable deviation from the specificembodiments shown herein may be made with- C9 in the spirit of theinvention. Accordingly, it is intended that the scope of the inventionbe limited only by the following claims.

We claim:

1. A method of electrostatically coating a workpiece comprising thesteps of applying, coating material to an endless belt, centrifugallyatomizing the coating material bypassing the belt through an arc at highvelocity, and electrostatically depositing the atomized coating materialon the workpiece.

2. A method of electrostatically painting a workpiece comprising thesteps of applying'paint to an endless belt, centrifugally atomizing thecoating material by passing the belt around a roller at high velocity,and electrostatically depositing the atomized paint on the workpiece.

3. A method of electrostatically coating a workpiece comprising thesteps of applying coating material to an endless belt, centrifugallyatomizing the coating material by passing the belt around a roller athigh velocity, passing the atomized material through a charging ring,and applying a high potential between the charging ring and theworkpiece for electrostatically depositing the atomized coating materialon the workpiece.

4. A method of electrostatically coating a workpiece comprising thesteps of applying coating material to an endless belt, centrifugallyatomizing the coating material by passing the belt around a roller at ahigh velocity, and applying a high potential between the belt and theworkpiece for electrostatically depositing the atomized coating materialon the workpiece.

References (Jited by the Examiner UNITED STATES PATENTS 2,307,698 1/43Melton et al. 7

2,681,725 6/54 Earnshaw 2392l8 2,687,803 8/54 Johnson 209127.1 2,723,64611/55 Ransburg 117--93.42 2,967,33 1' 1/61 Kaspar 118*629 X 3,059,61310/62 Nakaya 118624 20 RICHARD D. NEViUS, Primary Examiner.

JOSEPH REBOLD, Examiner.

1. A METHOD OF ELECTROSTATICALLY COATING A WORKPIECE COMPRISING THESTEPS OF APPLYING COATING MATERIAL TO AN ENDLESS BELT, CENTRIFUGALLYATOMIZING THE COATING MATERIAL BY PASSING THE BELT THROUGH AN ARC ATHIGH VELOCITY, AND ELECTROSTATICALLY DEPOSITING THE ATOMIZED COATINGMATERIAL ON THE WORKPIECE.